Computational screening and functional tuning of chemically stable metal organic frameworks for I2/CH3I capture in humid environments

Summary High chemical stability is of vital significance in rendering metal organic frameworks (MOFs) as promising adsorbents for capturing leaked radioactive nuclides, under real nuclear industrial conditions with high humidity. In this work, grand canonical Monte Carlo (GCMC) and density functional theory (DFT) methods have been employed to systematically evaluate I2/CH3I capture performances of 21 experimentally confirmed chemically stable MOFs in humid environments. Favorable structural factors and the influence of hydrophilicity for iodine capture were unveiled. Subsequently, the top-performing MIL-53-Al with flexible tunability was functionalized with different functional groups to achieve the better adsorption performance. It has been revealed that the adsorption affinity and pore volume were two major factors altered by the functionalization of polar functional groups, which collectively influenced the iodine adsorption properties. In general, this work has screened the chemically stable high-performance MOF iodine adsorbents and provided comprehensive insights into the key factors affecting I2/CH3I uptake and separation in humid environments.


INTRODUCTION
In contrast to traditional fossil fuels, nuclear energy is a promising green energy source to power the global economy, due to its emission-free and high energy density properties. 1,2However, the production of nuclear power comes with safety concerns. 3For instance, the leakage of volatile radioactive iodine nuclides (I 129 and I 131 ) in the nuclear accidents or during the used nuclear fuel reprocessing, poses serious threats to both the environmental safety and human metabolic health. 4Consequently, there is a pressing need to develop efficient methods to achieve iodine nuclides capture and separation for ensuring nuclear safety. 5,6The existing methods for volatile iodine nuclides removal include the precipitation, dry dedusting, wet scrubbing, adsorption, etc. 7 Among these methods, adsorption techniques using porous adsorbents have proved the superiority, which is attributed to their high removal efficiency, simple operation and design process, good system reliability and low maintenance costs. 8Various traditional porous materials have been tried and used for iodine adsorption, typically including the activated carbon and silver-exchanged zeolites (AgZ). 9,10However, the former exhibits the low separation efficiency and poor high-temperature resistance; while the latter have the drawbacks of low adsorption capacities and adverse impact on environment.All of these problems limited their practical applications. 52][13] The aforementioned attributes render the MOFs as promising candidates for iodine adsorption.However, the high humidity environment is quite common in the real nuclear industry and during the nuclear fuel reprocessing, 5 which proposed the higher requirement for both the chemical stability and competitive iodine adsorption property of MOFs. 14,157][18][19] Notable studies include the work by Nenoff et al., 16 who combined the simulations and experiments to analyze the competitive adsorption behavior of Cu-BTC in a mixed-gas stream ($1:1 ratio of I 2 :H 2 O vapor) at ambient pressure and 75 C.It was revealed that the Cu-BTC preferentially adsorbed I 2 over water (selectivity of I 2 /H 2 O = 1.5 and iodine uptake $175 wt %).Thallapally and co-workers also reported the I 2 adsorption research of SBMOF-1 and SBMOF-2 in the presence of humidity (33% RH and 43% RH) at room temperature. 17SBMOF-1 and SBMOF-2 showed 15 wt % and 35 wt % I 2 uptakes, respectively.Zhang's group conducted grand canonical Monte Carlo (GCMC) and density functional theory (DFT) methods to reveal the influence of H 2 O on different zeolitic imidazolate frameworks (ZIFs) during I 2 adsorption. 18They found that the water had a negative impact on I 2 uptake in hydrophilic materials due to the similar adsorption sites.Furthermore, besides the molecular form of iodine (I 2 ), radioactive organic iodides mainly the methyl iodine (CH 3 I) are also the important components of the volatile iodine nuclides and have been considered to be captured. 20,21For instance, Li's group constructed the MIL-101-Cr-TED and MIL-101-Cr-HMTA through post-synthetic modification to achieve the 35.2 wt % and 37.2 wt % uptake for CH 3 I at 150 C under humid conditions (RH = 81%). 22Yue et al. reported the generation of mesopores from ECUT-300, and enabled the corresponding adsorption capacity up to 0.85 g/g at 423 K under humid CH 3 I (RH = 50%). 23However, despite these advancements, the researches on MOFs for iodine adsorption in high humidity circumstance are still rare, much less the systematic comparison of the key factors (including the structural factors, hydrophilic properties, and modified functional groups) that influence the I 2 and CH 3 I adsorption performance.
In this work, we first identified 21 MOFs with outstanding chemical and water stabilities based on previous researches, 24,25 and conducted GCMC and DFT calculations to screen these high-performance MOFs for efficiently adsorbing and separating I 2 and CH 3 I in humid air environments; next, the structural factors and hydrophilic properties were explored to investigate the effect on iodine capture; finally, different types of functional groups were grafted onto the MIL-53-Al to achieve the enhancement of iodine adsorption performances and then the underlying impact mechanism was also further analyzed.

Computational screening of chemically stable MOFs
With reference to previous experimental studies, 21 representative MOFs with outstanding chemical and water stability have been initially selected. 24,25Relevant testing conditions and observations were shown in Table S1.GCMC simulations were performed to explore the adsorption and separation of gaseous I 2 and CH 3 I in MOFs at conditions of 423 K and 1 bar, using the RASPA package. 26423 K was the relevant operational temperature in the nuclear industry. 5,27,28To simulate the high humid environment during the actual reprocessing of the spent nuclear fuel, the mixed gas system was composed of 300 ppm I 2 (or CH 3 I), 68.5% N 2 , 18.4% O 2 , and 12.2% H 2 O (achieving a relative humidity of 100%). 5Simulation details including the MOF structures, interatomic potentials, simulation parameter settings, and adsorption selectivity formula were provided in supplemental information.The adsorption performance for I 2 and CH 3 I, as well as the geometric properties of the 21 chemically stable MOFs, were listed in Table 1.The considered structural factors encompassed the pore limiting diameter (PLD, 3.01-28.26A ˚), largest cavity diameter (LCD, 4.8-33.23A ˚), void fraction (0.131-0.852), accessible surface area (S cal ; 186.03-3594.65m 2 /g), and pore volume (V p-cal ; 0.087-1.744cm 3 /g).The relatively high surface area and pore volume indicated these materials as great potential adsorbents for I 2 and CH 3 I adsorption.The theoretically calculated surface area and pore volume (S cal and V p-cal ) could be roughly comparable to the values extracted from previous experiments (S exp and V p-exp ).Nonetheless, disparities existed, likely stemming from the presence of impurities or defects in the actual materials.Notably, for certain selected MOFs (e.g., UiO-66, UiO-66-NH 2 , JUC-110, etc.), the PLD was smaller than the kinetic diameter of CH 3 I molecules (4.23 A ˚), rendering them unsuitable for evaluating CH 3 I adsorption performance.It was evident that the uptake amounts of CH 3 I in MOFs were overall much lower than those of I 2 .This discrepancy arose from the comparatively inert chemical reactivity of CH 3 I, 9 making the separation and immobilization of CH 3 I more challenging compared to that of elemental iodine.

Relationships between structures and adsorption performance
In order to further vividly reveal the effect of structural factors on adsorption performance, the structure-property relationships were illustrated in Figure 1.For I 2 and CH 3 I adsorption, Figures 1A and 1B showed that the optimal LCD and void fraction were around 7.1 A ˚and at the range of 0.3-0.7,respectively; Figures 1C and 1D indicated that the optimal surface area and pore volume were at the range of 450-2230 m 2 /g and 0.2-0.9cm 3 /g, respectively.The aforementioned four structural factors were correlative with each other (e.g., the larger LCD were more likely to induce the higher void fraction); but LCD exhibited more obvious and greater impact on adsorption performance, by causing the constricted porosity and compact interaction between the MOFs and iodine molecules.The aforementioned results were well consistent with the previous researches. 47We picked the several top-performing materials and marked their names in  2G.The Q st of I 2 in SNU-80 outperformed all other selected MOFs, but its I 2 uptake amount (22.51 cm 3 /g) was lower than other MOFs.The reason might lie in its high Q st of H 2 O (34.8 kJ/mol), which led to the more drastic competitive adsorption between the H 2 O and I 2 and therefore reduced I 2 selectivity (2.95 3 10 4 ) in SNU-80.Conversely, Zn(1,3-BDP) and MIL-53-Al with the lowest Q st of H 2 O of 16.5 kJ/mol and 13.6 kJ/mol, exhibited the top-two I 2 uptake amounts and selectivity.Consequently, it could be concluded that the Q st of H 2 O had a significant impact on I 2 adsorption due to competitive adsorption in MOFs.However, compared to I 2 adsorption, the influence of Q st of H 2 O on CH 3 I adsorption was not so remarkable; whereas the Q st of CH 3 I played the main role.Al-PMOF with the highest Q st of CH 3 I of 69.4 kJ/ mol, owned relatively high Q st of H 2 O (36.8 kJ/mol), but its CH 3 I uptake amounts (5.56 cm 3 /g) and selectivity (3.01 3 10 3 ) were still the highest

Functional tuning of MIL-53-Al
In pursuit of the better adsorption performances, MIL-53-Al was picked out to tune the surface functionality, due to its excellent comprehensive performance including the high I 2 and CH 3 I uptake amounts, extraordinary hydrophobicity (with the lowest Q st of H 2 O) and the most critically-facile modification.As shown in Figure 3   Subsequently, adsorption simulations in the mixed gas system were applied to elucidate the trace I 2 and CH 3 I adsorption performance.The mixed gas composition was set as the same conditions as before in the investigations of initial 21 chemically robust MOFs.The adsorption performance and geometric properties of functionalized MIL-53-Al-X were shown in Table 2; and the isosteric heat of adsorption for guest gas molecule was plotted in Figure S3.It was noteworthy that the Q st of H 2 O (ranging from 13.1 kJ/mol to 17.5 kJ/mol) in functionalized MIL-53-Al-X series maintained the low level with relatively small energy differences, and therefore had no longer played the obvious role in I 2 uptake amounts.This phenomenon was different from the previous six top-performing materials (i.e., JUC-110, Zn(1,3-BDP), NOTT-300, Al-PMOF, SNU-80, and MIL-53-Al).Table 2 exhibited that the functionalization of the polarized functional groups did enhance the I 2 selectivity (except the MIL-53-Al-PYDC), but the I 2 adsorption amounts did not have the obvious enhancement.For researching the reason, we analyzed the influence of different structure factors of MIL-53-Al-X series on I 2 adsorption capacity in Figure S4, in which an obvious correlation was observed between the pore volume and I 2 uptake amount.So, it was attributed to the smaller pore  volume and bigger steric hindrance that limited the I 2 uptake in functionalized MIL-53-Al-X series, despite the enhanced the I 2 selectivity.Figure 4A illustrated that the pore volumes (0.37-0.62 cm 3 /g) were a major determining factor, whereas the Q st of I 2 (59.4-71.9kJ/mol) played a relatively minor role in influencing I 2 uptake.For instance, MIL-53-Al-Cl and MIL-53-Al-Br had the relatively high I 2 selectivity of 2.80310 5 and 3.75310 5 , considerably higher than that of unfunctionalized MIL-53-Al (1.60310 5 , 49.85 cm 3 /g).However, their small pore volumes (0.463 cm 3 /g and 0.369 cm 3 /g) resulted in the lower I 2 adsorption amounts of 44.96 cm 3 /g and 36.72 cm 3 /g, respectively.
Conversely, MIL-53-Al-NH 2 and MIL-53-Al-CH 3 with relatively large pore volumes (0.509 cm 3 /g and 0.470 cm 3 /g) exhibited the top-two I 2 uptake amounts of 50.65 cm 3 /g and 49.74 cm 3 /g, respectively.Regarding MIL-53-Al-PYDC, the largest pore volume (0.617 cm 3 /g) was balanced with the lowest I 2 affinity (59.4 kJ/mol), resulting in a mediocre CH 3 I uptake amount of 41.07 cm 3 /g.Additionally, the good correlation shown in Figure 4B demonstrated that the Q st difference of I 2 against other adsorbates governed the I 2 adsorption selectivity in MIL-53-Al-X series.With regard to CH 3 I adsorption, as shown in Table 2 and Figures 4C and 4D, the modified functional groups significantly enhanced the CH 3 I adsorption performances, including both CH 3 I uptake amounts and selectivity.The CH 3 I uptake amounts and selectivity were positively correlated with the Q st of CH 3 I, indicating the affinity for CH 3 I played a major role in CH 3 I uptake in MIL-53-Al-X series.The impact of the functionalization-induced reduction in pore volume was nearly negligible (Figure S5).MIL-53-Al-CH 3 , exhibiting the highest Q st for CH 3 I of 71.95 kJ/mol, achieved the highest CH 3 I uptake amount of 2.8 cm 3 /g, which was more than twice that of the pristine MIL-53-Al (1.19 cm 3 / g); followed by the MIL-53-Al-NH 2 with the CH 3 I uptake amount of 1.81 cm 3 /g.Figures 5A-5D presented the GCMC-simulated adsorption density plots of I 2 , CH 3 I, and H 2 O during the competitive adsorption.It could be observed that the I 2 and CH 3 I molecules were primarily concentrated near the center of pore channels, whereas the H 2 O molecules were mainly located near the backbones of the framework.The above differences of the density contour shapes could be attributed to the varying uptake amounts and molecule sizes.The larger space near the center of pore channels allowed for accommodating a greater number of larger molecules (e.g., I 2 and CH 3 I).For H 2 O molecules, the sites near the Al clusters or methyl appeared to exhibit a higher affinity, likely due to the strong hydrogen bonding interactions.To understand the aforementioned phenomenon at the molecular scale, DFT calculations were applied to determine the corresponding binding energies between the MOF and individual molecule (I 2 , CH 3 I, N 2 , O 2 , or H 2 O) at four possible adsorption sites, depicted in Figure 5E: near the Al cluster, above the benzene ring, around the methyl (-CH 3 ), and at the center of the pore channel.The binding energies were detailed in Table S4, and the optimal adsorption situations with the highest binding energies for each molecule were shown in Figure 5F.The optimal adsorption positions for I 2 (65.90 kJ/mol) and CH 3 I (47.86 kJ/mol) were slightly off-center from the channel; the smaller N 2 and O 2 molecules could be adsorbed near the Al clusters, benzene ring and methyl due to their smaller steric hindrance; the adsorption situations for H 2 O were complex: Al clusters were the optimal adsorption sites with the highest binding energy of 40.52 kJ/mol, followed by the methyl (34.16 kJ/mol), the benzene ring (23.25 kJ/mol), and the center of the pore channel (12.45 kJ/mol).Importantly, the hydroxyl functional groups connected to Al clusters played a significant role in hydrogen bonding interactions to achieve a high binding energy.Additionally, the binding energies of pristine MIL-53-Al at different adsorption sites were also calculated (Table S5), conforming the methyl functionalization really enhanced the adsorption of I 2 and CH 3 I.These DFT calculations also confirmed the validity of the GCMC simulation results.

Conclusions
In summary, we have performed GCMC and DFT calculations to screen 21 experimentally confirmed chemically stable MOFs and identified MIL-53-Al as a promising iodine adsorbent under high humidity circumstances.Structural factors have a significant effect on adsorption performance for both I 2 and CH 3 I; whereas hydrophilicity had a more pronounced impact on I 2 adsorption behaviors compared to CH 3 I.The introduction of polar functional groups proved to substantially enhance the iodine adsorption selectivity by promoting the adsorption affinity.However, this enhancement came at the cost of reduced pore volume, particularly affecting the uptake capacity for I 2 adsorption.Overall, MIL-53-Al-CH 3 outperformed other MOFs, exhibiting notably improved CH 3 I uptake (2.8 cm 3 /g) and selectivity (3.62310 3 ), while simultaneously maintaining excellent I 2 uptake (49.74 cm 3 /g) and selectivity (4.99310 5 ).Moreover, at the molecular level, it was revealed that I 2 and CH 3 I molecules were primarily adsorbed near the channel center in MIL-53-Al-CH 3 ; whereas other competitive gas molecules tended to be located around the backbones of the framework.Generally, this work has provided a feasible screening and design approach for promising high-performance iodine adsorbents in nuclear waste managements under high humidity conditions.

Limitations of the study
Our work has theoretically screened and studied the chemically stable high-performance MOF as iodine adsorbents in humid environments.However, the experimental investigation and validation on the iodine adsorption performance needs to be further explored.supercells were applied to ensure the system size to be twice longer than the cutoff distance (12 A ˚).In addition, the selectivity of I 2 and CH 3 I during adsorption was calculated as the following equation: 8,28 selectivity iodine = X iodine =Y iodine X others =Y others where X iodine and Y iodine denoted the uptake amounts and gas phase concentration of targeted gas iodine (I 2 or CH 3 I), respectively; X others and Y others were the uptake amounts and gas phase concentration of other gas components (N 2 , O 2 and H 2 O).

Density functional theory calculations
9][60][61] The cut-off energy for the plane wave basis was set as 500 eV.DFT-D3(BJ) dispersion corrections were included to consider the van der Waals interactions. 62,63Besides, the binding energy between the MOF and guest gas molecule was determined through the following equation: where E coordination stood for the optimized energy of the coordinated structures after fully structure relaxation, E MOF and E guest were the energy of the individual MOF and guest molecule, respectively. 19

Figure 1 .
Figure 1.Relationships between structures and adsorption performance (A and B) (A) I 2 and (B) CH 3 I uptake amounts vs. largest cavity diameter colored by void fraction.(C and D) (C) I 2 and (D) CH 3 I uptake amounts vs. surface area colored by pore volume.
, different types of functional groups were grafted onto the benzene ring of MIL-53-Al to get MIL-53-Al-X series, in which MIL-53-Al-H represents the pristine MIL-53-Al without functionalization.After the fully structural relaxation using DFT calculations, theoretical geometric properties of MIL-53-Al-X series were listed in Table 2.The adsorption isotherms of the pure gaseous I 2 , CH 3 I, N 2 , O 2 , and H 2 O in MIL-53-Al-X series at 423 K and 0-1 bar were simulated and plotted in Figure S2.The I 2 and CH 3 I adsorption isotherms in the MIL-53-Al-X series featured type I micropore filling adsorption mechanism with the uptake saturated at relatively low pressure.The low H 2 O uptake amount (below 3 cm 3 /g even at 1 bar of pure H 2 O) shown in H 2 O adsorption isotherms indicated the MIL-53-Al-X series as the good hydrophobic materials.

Figure 4 .
Figure 4. Adsorption performance studies of functionalized MIL-53-Al-X series (A and B) Correlations between the Q st (and pore volume) and adsorption capacity of (A) I 2 and (B) CH 3 I in MIL-53-Al-X series.(C and D) Correlations between Q st difference of (C) I 2 and (D) CH 3 I against other adsorbates and adsorption selectivity in MIL-53-Al-X series.

Figure 5 .
Figure 5. Adsorption location studies in MIL-53-Al-CH 3 (A and B) Adsorption density plot pictures of (A) I 2 and (B) H 2 O during competitive I 2 adsorption.(C and D) Adsorption density plot pictures of (C) CH 3 I and (D) H 2 O during competitive CH 3 I adsorption.(E and F) (E) Simulated adsorption locations for DFT calculations and (F) DFT-optimized optimal geometric positions of different molecules in the MIL-53-Al-CH 3 pore channel.

Table 1 .
Adsorption performance and geometric properties of chemically stable MOFs

Table 2 .
Adsorption performance and geometric properties of functionalized MIL-53-Al-X